Universal Containers (UC) calculates commissions on their Opportunities in different ways base on complex rules that vary depending on the line of business of the Opportunity. Whenever a new line of business is added to Salesforce at UC, it is likely that a different calculation will need to be added too. When an Opportunity's stage is changed to Closed/Won, its commission should be calculated in real time. What should a developer use so that different implementations of the commission calculation can be invoked on the stage change?